Laser-beam powder sintering technology is used to manufacture three-dimensional objects, such as prototypes or models but also functional parts, in particular in the automobile, nautical, aeronautical, aerospace, medical (prostheses, auditory systems, cell tissues, and the like), textile, clothing, fashion, decorative, electronic casing, telephony, home automation, computing or lighting fields.
A thin layer of powder of the polymer in question is deposited on a horizontal plate maintained in a chamber heated to a certain temperature. The laser supplies the energy required to melt the powder particles at various points of the layer of powder in a geometry corresponding to the object, for example using a computer that stores the shape of the object and that reproduces this shape in the form of slices. Next, the horizontal plate is lowered by a value corresponding to the thickness of one layer of powder (for example between 0.05 and 2 mm and generally of the order of 0.1 mm), then a new layer of powder is deposited. This layer of powder is at a temperature referred to subsequently as the temperature of the powder bed (or temperature of the bed). The laser supplies the energy required to melt the powder particles in a geometry corresponding to this new slice of the object and so on. The procedure is repeated until the entire object has been manufactured. Besides the melting of the powder particles induced by the energy supplied by the laser, it is necessary to use conditions that enable the coalescence of the particles with one another and a good adhesion/coalescence of the layers with one another so that the mechanical properties of the objects manufactured are maximized. One determining parameter in obtaining parts with optimal properties is the temperature of the powder bed.
To date, the search for the optimal temperature of the powder bed for a given polymer used in laser sintering goes through sintering tests then through an analysis of the parts obtained (density, porosity, mechanical properties, etc.). Analysis of the parts makes it possible to single out the best experimental parameters and improvement paths for future tests aiming to determine the optimal parameters. It is therefore necessary to carry out several tests under real conditions for producing an object (at various temperatures of the powder bed in particular) in order to determine the conditions that allow the manufacturing of parts with maximized properties. This takes time and is expensive both regarding machine and operator time or else consumption of polymer powder.
The methodology used in the process that is the subject of the present invention makes it possible to understand the behavior of the material under the conditions of the polymer powder melting process and to select, for a given material, the process conditions (in particular the temperature of the powder bed) in order to ensure a coalescence of the particles with one another and between the successive layers.
It is well known to a person skilled in the art that the transformation window (temperature range of the powder bed) is between the crystallization temperature (Tc) and the melting temperature (Tm) of the polymer considered in the case of semicrystalline polymers. In the case of amorphous polymers, the transformation window lies above the glass transition temperature (Tg) as measured by differential thermal analysis (DSC).
In the case of semicrystalline polymers, if the temperature of the powder bed is too close to Tm, then there is agglomeration (caking) of the powder outside of the zone constituting the object, hence a loss of material. Furthermore, the molten polymer risks being too fluid and may flow beyond the first layer being formed or beyond the contours.
If the temperature of the powder bed is too close to Tc, then there is deformation (curling) of the part, due to the excessively rapid crystallization of the successive layers of the material.
In the case of amorphous polymers, the temperature of the powder bed should be above the Tg. In this case, the absence of crystallization makes it possible, on the one hand, to avoid the deformation of the parts and, on the other hand, to limit the phenomenon of shrinkage of the parts during the cooling phase.
If the temperature of the powder bed is too far above the Tg, there is agglomeration (caking) of the powder outside of the zone constituting the object, hence a loss of material. Furthermore, the molten polymer risks being too fluid and may flow beyond the first layer being formed or beyond the contours.
Outside of a particular temperature range of the polymer powder bed, the quality of the part obtained is not right.
DSC studies therefore make it possible to set a temperature range (between Tc and Tm for semicrystalline polymers and above Tg for amorphous polymers) in which it is then necessary to choose, via successive machine tests and via an analysis of the parts obtained, the appropriate temperature of the powder bed. In addition to providing just a temperature range, these DSC tests only very partially take into account the specificities of the products, for example their molecular weight.
In this laser sintering process, the portion of the powder bed affected by the layer in the process of being sintered, after fusion by the laser, rapidly returns to the temperature of the powder bed. It is therefore this temperature that controls the viscosity of the molten material, which key parameter governs the coalescence of the particles with one another and between the successive layers. Moreover, this adapted viscosity should last a certain time in order to obtain a coalescence of the particles with one another and between the successive layers. This time is defined as “the open time”. Specifically, in the case of semicrystalline polymers, cooling induces a crystallization which leads to an increase in the viscosity and, beyond a certain viscosity value, the successive layers will no longer be able to coalesce (since the excessively high viscosity will become an impediment to the coalescence).
The applicant has observed that the conditions allowing a good adhesion and a good coalescence between the powder particles on the one hand, and between the layers of coalesced powders on the other hand, may be understood with the aid of a rheological analysis. This analysis makes it possible to precisely establish the ideal temperature range condition of the powder bed that makes it possible to prepare objects by melting polymer powders and in particular the laser sintering of polymers. It therefore makes it possible to establish the temperature range from the viscosity of the melted layer and from the open time allowing a good coalescence of the particles with one another and between the successive layers.